1. Field of the Invention
The present invention relates to cermets (ceramic/metal composites) and ceramic/glass composites, and more particularly to cermets and ceramic/glass composites containing silicon nitride as their major ceramic phase.
2. Description of Related Art
Silicon nitride ceramics possess a number of physical properties that make them useful in a variety of tribological and high temperature applications. These properties include good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance, high electrical resistivity, resistance to chemical attack, and convenient fabrication into near-net shapes. Two of the many uses of silicon nitride ceramics are as cutting tools, and as parts for pumps and engines.
Unfortunately, silicon nitride ceramics have certain shortcomings. They are often brittle. They also have a fracture toughness and strength lower than might be desired for various applications. This may be due to structural flaws introduced by conventional manufacturing techniques.
Numerous, less than completely successful, approaches to overcome the shortcomings are known. One approach pressure infiltrates a porous (rather than densified), particulate silicon nitride preform with another material, such as a metal or glass. This approach typically requires long process times in exchange for less than desirable improvements in physical properties. A second approach involves forming a preform from silicon nitride whiskers rather than silicon nitride powder. This approach yields preforms that are only 30 to 40 percent dense. In other words, the whiskers occupy only that volume of the preform. The whiskers, after processing into a preform, typically have low aspect ratios. If mixing times are too short, the preforms tend to have a nonuniform distribution of whiskers and a nonuniform density. In addition, whiskers are susceptible to moving with respect to each other during infiltration. This approach also usually mandates use of special equipment and observance of procedures for handling small whiskers during preform preparation. In addition, silicon nitride whiskers are expensive. A third approach employs chopped silicon nitride fibers instead of the much finer whiskers. This approach fails to substantially lower cost and introduces further limitations such as lack of isotropic uniformity and low reinforcing material content. In addition, polycrystalline fibers do not have the same high strength characteristics as whiskers.